# Why do physical units have different prefixes than the ones used for counting? [closed]

For counting, we use thousand, million, billion, trillion etc. For physical quantities, thousand becomes Kilo, million is Mega and (short scale) billion is Giga. In both these systems, the consecutive prefixes differ by an order of $10^3$. For currency, the first set of prefixes is used but for physical quantities, the second set is used. Why use two different systems?

## closed as off-topic by ACuriousMind♦Apr 15 '17 at 15:38

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• I'm voting to close this question as off-topic because asking for the reasons for notation/terminology is off-topic. If you are interested in a historical perspective, consider asking at History of Science and Mathematics instead. – ACuriousMind Apr 15 '17 at 15:38
• @ACuriousMind Perhaps a moderator could migrate the question to HSM SE. – Kunal Pawar Apr 15 '17 at 15:40
• @KunalPawar As currently written, the question is asking for a "reason" (which may well be ahistorical, i.e. the reason to keep the system today might be wholly different from why it was historically introduced) rather than a historical account, so I will not migrate it, since it is not clear that the asker is interested in the history as such at all. If Dove wants to know about the history I'd just advise them to ask a question on hsm.SE themselves, making clear what parts of the historical origin of the SI unit system they are specifically interested in. – ACuriousMind Apr 15 '17 at 16:10
• @ACuriousMind Fair enough. – Kunal Pawar Apr 15 '17 at 16:33

This is speculation, but some advantages of the SI system are:

1. It's much easier to distinguish "kilo" from "milli" than "thousand" from "thousandth", and easier to distinuguish "mega" from "micro" than "million" from "millionth".

2. In writing, "kg" is much quicker to write than "thousand grams". We could maybe still make an abbreviated system based on "thousand" and "million" but we'd need a way to distinguish "thousand" from "thousandth" and so on, probably resulting in somewhat arbitrary abbreviations like "k" and "$\mu$" being used. Then we'd likely naturally make up names for these abbreviations for when we read them aloud. And we might end up with something very like what we have.

3. "kg" means the same thing in every language, but "thousnd grams" would be written differently in English, German, French, Chinese, etc. So the system aids international communication between scientists.

Further, if you consider Chinese and Japanese (and probably other languages I know less about), they don't traditionally divide their numbers every 3 decades but every 4. In Chinese, 10,000 isn't "ten thousand" but its own number, "wan". 100,000 is not "one hundred thousand" but "ten wan". Using SI gives a regular system across cultures.

• +1. Yeah, but then we should adopt the kilo, mega, giga system for currency and counting too. You showed that SI has advantages. – Dove Apr 15 '17 at 15:50
• @dove, you've never heard somebody talk about "megabucks"? – The Photon Apr 15 '17 at 15:54
• No, I always heard grand or thousand instead of kilo. And, we always use million not mega. Or maybe I've just not heard. – Dove Apr 15 '17 at 15:56
• FYI, but realistically the reason this isn't used in finance is that finance worked out their own jargon long before SI came along. – The Photon Apr 15 '17 at 15:58